and a subsequent install using GDebi Package Installer, it keeps coming back with my old bookmarks so I know that the removal was not clean.

I am attempting to reinstall

opera-stable_51.0.2830.34_amd64.deb

with GDebi Package Installer.

I know that the issue is with the configuration of Opera on this specific machine because: 1) Firefox works from this machine with no problems, and 2) Opera is working find from a different machine on the same LAN.

I'm looking for help to either:

Completely remove an existing Opera install including all configuration files, or

Fix whatever is broken in my installation's configuration files.

Thank you for any help. I've been fighting this for a few weeks on and off.
Mike

Under /home/<yourhomedir>/.config/
Just rename opera directory to something else. Then give it a shot.
It will automatically recreate the opera dir but you can see if it works with the newly created dir.
Then if you want to give the original directory another try, rename it back to opera after deleting the newly created one.

First, though you should try opera://settings/clearBrowserData, check every box but passwords and see if that helps. Also leave VPN or Turbo off until you get it working normally.